1. Field of the Invention
This invention relates to the refining of metal and alloy, and more particularly to a method of dephosphorizing a metal, or an alloy containing more oxidizable elements than iron such as Cr, Mn, Si, Al, Mo, Zr, Nb, V and/or W, for example, ferroalloys and alloy steels.
2. Description of Prior Art
Conventionally the oxidizing refining has been employed to achieve dephosphorization of a molten metal or alloy. This is performed by oxidizing the molten metal under a basic slag to convert phosphorus to P.sub.2 O.sub.5 which is then converted to Ca.sub.3 (PO.sub.4).sub.2 in the slag and fixed therein.
While the above-described prior art has capabilities which are comparatively well suited for dephosphorization of molten pig iron and molten steels of ordinary composition, it is not applicable to processes for dephosphorizing metals and alloys containing more oxidizable elements than iron such as Cr, Si, Al, Mn and the like in an amount of about 4% or more based on the total weight of the metal or alloy, because these valuable elements tend to be oxidized in preference to phosphorus with the resulting yields of these elements being lowered. In addition thereto, as the oxidized portion of the molten metal is transferred to the slag, the dephosphorization is retarded to remarkably large extent physically as well as chemically. For this reason, it has been conventionally considered in the art that it is impossible to achieve a substantial level of dephosphorization of high chromium ferrous metals such as stainless steels and ferrochromium.
Recently, however, many efforts have been devoted to the investigation of effects of phosphorus involved as an impurities in metal materials, and it has been found that, particularly in the case of high chromium steels such as stainless steels, the phosphorus adversely affects the workability, corrosion resistance and weldability. In recent years, the increased phosphorus content in commercially available chrome ore and coke and the demand of lowered phosphorous contents in the final products leads to strong demand of making it possible to achieve the dephosphorization of the molten high chrome steel which has heretofore been impossible.
It is also known to use a plag containing metallic calcium along for the purpose of dephosphorizing a molten metal or molten alloy as disclosed in Japanese Laid-open Patent Specification No. 49-35213 and Japanese Patent Publication No. 51-2402. This method utilizes the following reaction: EQU 3Ca(in slag)+2P (in metal).revreaction.Ca.sub.3 P.sub.2 (in slag) (1)
so that the dephosphorization can be carried out without causing oxidation of the molten metal. When this method is applied to processes for refining molten metals on a large scale, however, certain drawbacks must be overcome. Metallic calcium is very expensive because of its low energy efficiency and productivity, and is also very reactive so that the handling of metallic calcium in large quantities is very troublesome.